A wavelength division multiplexing optical transmission system having a plurality of pieces of node equipment connected is an optical transmission communication system that increases the transmission capacity of signals transmitted through a single fiber by wavelength multiplexing signal light waves assigned to a plurality of wavelengths by using wavelength division multiplexing.
The wavelength division multiplexing is a communication method that applies the fact that signal light waves with different wavelengths can exist independently, and can increase the transmission capacity easily by increasing the number of wavelengths to be subjected to the wavelength multiplexing. In addition, it can carry out multiplexing/demultiplexing of a particular wavelength without affecting the other wavelengths.
Furthermore, thanks to research and development of optical transmission technology and optical devices, long-distance transmission and an increasing number of wavelength multiplexing are realized by utilizing optical amplification technique as typified by EDFAs (Erbium-Doped Fiber Amplifiers) capable of achieving broadband and high gain optical amplification of wavelength division multiplexing signals as they are without any electrical conversion and by utilizing high multiplexing number and low loss wavelength multiplexing/demultiplexing devices as typified by arrayed-waveguide gratings (AWGs).
Such a wavelength division multiplexing optical transmission system comprises an optical amplification functional unit for compensating for a loss of optical power levels caused through a transmission path and various optical devices; a wavelength multiplexing/demultiplexing functional unit for carrying out multiplexing/demultiplexing of wavelengths; a transponder functional unit that has a function of converting a signal received from a downstream device to a signal light wave with a wavelength assigned to the wavelength division multiplexing optical transmission system and transmits to a wavelength multiplexing/demultiplexing functional unit, and a function of receiving a demultiplexed signal light wave from the wavelength multiplexing/demultiplexing functional unit and converting it to a signal for a downstream device and transmitting to the downstream device; and a monitoring control unit for monitoring the functional units.
In addition, as a network becomes complicated, demands on the wavelength division multiplexing optical transmission system are growing. Thus, it copes with the demands by incorporating a dispersion compensation functional unit for suppressing signal degradation due to dispersion in a fiber, a switch functional unit for carrying out in the wavelength multiplexing/demultiplexing functional unit the multiplexing (Add)/demultiplexing (Drop)/transmission (Through) of a wavelength to deal with a network path change request from a remote region, and a DGE (Dynamic Gain Equalizer) functional unit for equalizing optical power levels of the individual wavelengths.
The wavelength division multiplexing optical transmission system with such a configuration is constructed on the assumption that the signal light waves assigned to the individual wavelengths are independent and that a fault of a signal light wave of a different wavelength does not affect the signal quality of signal light waves having nothing to do with the fault.
However, as for a fault of the optical amplification functional unit for amplifying the wavelength division multiplexing signals as a whole or that of the wavelength multiplexing/demultiplexing functional unit, or a physical phenomenon such as nonlinear optical effects occurring in a transmission path connecting between two or more pieces of node equipment, they can sometimes affect all the wavelengths and degradation can sometimes occur in signal light waves having nothing to do with the fault. Accordingly, it is necessary to manage signal degradation factors rightly in a system design and construction, and a mechanism is required for preventing a fault, even if it occurs, from affecting the signal quality of signal light waves with wavelengths having nothing to do with the fault.
Accordingly, to ensure long-distance transmission and high transmission quality, the wavelength division multiplexing optical transmission system manages optical devices such as an optical amplifier and variable optical attenuator in the optical amplification functional unit rightly by carrying out feedforward or feedback control in such a manner as to adjust the optical power level appropriately for each wavelength. In particular, to increase quality of maintenance and operation, the system is usually divided into several pieces of node equipment or packages in accordance with individual functional blocks, and sometimes undergoes feedback control to control optical power levels over the packages.
Accordingly, as for connection optical cords that connect between the packages, even if the loss of their optical power levels increases because of fiber microbending (a sharp reduction of the radius of curvature of a fiber), sticking of a foreign body to an end face of an optical connector or a half fit of the optical connector, the system can automatically adjust the gain of the optical amplifier or the loss of a variable optical attenuator through the feedback control to achieve a target optical power level which is a management value.
On the other hand, there are some cases where a package is exchanged during system maintenance, and a half fitting state or a half extracted state of an optical connector (a state in which the optical connector is pulled out halfway) can occur in a connection optical cord that connects between the packages in an upstream device or package of the DGE functional unit, in which case a maintenance worker often notices the half extracted state after that and fits the optical connector completely again to correct it.
In addition, as a wavelength division multiplexing optical transmission system, there are a Point-to-Point system and a ring system based on an OADM (Optical Add Drop Multiplexer) that Add/Drop/Through only any given wavelength in light as it is. Since the ring system based on the OADM causes its component, an OADM device (referred to as “node equipment” from now on), to carry out Through setting and Add/Drop setting for each wavelength, the individual settings are mixed together in operation. In a ring configuration, when a fault occurs in a transmission path or upstream node equipment, even though a signal light wave with a wavelength passing through the faulty place has a fault, a signal light wave with a wavelength that does not pass through that place should be free from a fault.
However, when an optical connector in a half extracted state between packages in the upstream are inserted again, only a signal light wave with a Through set wavelength (Through wavelength) has an instantaneous level increase in the output of the DGE functional unit. On the other hand, since a signal light wave with an Add set wavelength (Add wavelength) is not affected by the reinsertion of the optical connector, it maintains its normal optical power level. Accordingly, a transmission light amplifier placed downstream of the DGE functional unit has an optical power level difference between the input signal light wave with the Add wavelength and the input signal light wave with the Through wavelength, and the Through wavelength becomes an excessive input state.
On the other hand, although the transmission light amplifier can amplify a signal light wave as it is by using an excitation laser diode, its output level has a saturation level (Psat_th) as its total power and it cannot output power beyond that.
Accordingly, when the total power becomes an excessive input state for the transmission light amplifier, gain saturation occurs in the transmission light amplifier. Thus it cannot maintain the gain of the individual wavelengths at a constant value, and has a phenomenon of reducing the gain of the shortwave side optical output power level (optical power level of the signal light wave with the Add wavelength) markedly.
When such an instantaneous optical power level reduction occurs, a signal receiver unit on the opposite side cannot maintain the signal quality and can cause an information loss. Thus, it has a problem of affecting the signal quality of signal light waves with wavelengths other than those at the faulty spot when the optical connector is reinserted.
Considering this, a wavelength division multiplexing optical transmission system disclosed in Patent Document 1 is configured in such a manner that it comprises an optical power level adjusting unit in each receiver unit after demultiplexing the wavelength division multiplexing signals, and causes the optical power level adjusting unit to control the level when fluctuations occur in the wavelength number.